Studies of the molecular basis of human severe combined immunodeficiency (SCID) have ushered in a new era of improved diagnosis and treatment of these rare, but serious genetic disorders and also contributed to the fields of immunology and lymphocyte development. While many disease genes for human SCID are now known, less is known about leaky or partial SCID, also called combined immunodeficiency (CID); both SCID and CID cases of unknown genotype provide opportunities for further discovery. In addition, the mouse offers the power to dissect lymphoid developmental pathways, and we have discovered a new form of mouse SCID, deficiency of a predicted transcription factor Zbtb1. Mice lacking Zbtb1 have no T cells, but B lineage development is only mildly impaired. This T-B+ SCID model is important because its phenotype resembles that of humans with cytokine signaling pathway defects in the common g chain (gc) receptor, the IL-7 receptor a chain (IL7Ra), and Janus kinase 3 (Jak3), whereas the mouse knockouts lacking these proteins differ from humans in having T cells, but lacking B cells (T-B+). We will combine resources from human patients and mouse models to address the pathogenesis of lymphocyte developmental and functional defects. Samples from patients with SCID/CID will be assessed for defects in known SCID genes, and the cases remaining without a genotype assignment will be studied for defects in new gene candidates. Meanwhile, we will define the specific developmental and functional lymphoid phenotype of Zbtb1 deficient mice to uncover the normal role of Zbtb1 in mouse T and NK cell development and B cell differentiation and function. Comparisons between the Zbtb1- mouse, humans with cytokine signaling defects, and normal human hematopoietic stem cells (HSC) in which ZBTB1 gene expression is knocked down by RNAi will provide new insight into lymphoid development. Thus the goals of this proposal are to (i) assemble samples from SCID and CID patients without known gene defects; (ii) explore SCID pathogenesis of B cell impairment in Zbtb1- mice; (iii) use parallel in vitro and in vivo strategies to compare lymphoid developmental potential of HSC from wild type vs. Zbtb1 knockout mice and normal vs. ZBTB1 knockdown cord blood stem cells; and (iv) find Zbtb1 interacting partners and gene targets, which (along with ZBTB1 itself) will be assessed for mutations that may cause human SCID.